The present invention relates to a method of producing high yield pulps at high brightness for papermaking, that having high requiring a relatively low cost plant, and also to a pulp havinghigh yield with high unbleached brightness and good physical properties.
The increasing demand for paper and products based on cellulose requires a more complete wood utilization, and chiefly an increased hardwood use. Such utilization can be achieved with the high yield pulps with good mechanical properties and high brigthness of the present invention. Furthermore, highly costly plants meet financial difficulty and require high capacity of output to otain an economical production of pulps.
The need of big operation units for pulp production creates difficulty in the development of the paper industry in the underdeveloped countries and hinders the complete utilization of forest resources even in highly industrialized nations. The production of pulps with methods requiring relatively low cost plants is an important factor in the future development of the paper industry.
It is known that it is possible to obtain high yield pulps (85-95%) by treating wood chips (particularly hardwoods) with an aqueous solution of sodium hydroxide that has a cooking action on the wood more favorable than other chemicals. The wood swells rapidly when treated with caustic soda solution. The cross-linked structure of lignin limits the swelling of the whole fiber. Because the swelling differential creates stresses within the structure, much of the highly lignified outer layers are shed by the fiber when the treated wood is fiberized in a disk refiner. The exposed surface provides a good interfiber bonding.
The soda caustic solution saponifies the acetyl groups associated with the hemicelluloses and the esters and lactone groups that probably bond the hemicelluloses to the lignin; all of these facts increase the swelling of the cell walls and facilitate separation of fibers and interfiber bonding. Temperature higher than room temprature increases the rate of penetration of alkaline solution into the chips and speeds the above mentioned reactions of alkali on wood chips giving a pulp with improved properties.
The alkali treatment of the wood produces colored pulps, their colors varying from pale yellow to dark brown.
The difficult brightening of these high-lignin pulps increases the cost of plant and pulp production.
In the past, chemical processes have been used which employ an alkali metal borohydride in an effort to improve the brightness of a pulp. However, such prior processes have generally been carried out at relatively high temperatures in excess of 160.degree. C for relatively long periods of time which affects the life and effectiveness of the borohydride for achieving maximum brightness, but which also dissolves a considerable quantity of the lignincellulosic material, therefore resulting in a lower yield (fibers vs. pulpwood). Such process is found, for example, in the U.S. Pat. No. 3,042,575 to Hartler.
Also, an alkali metal borohydride has been utilized in conjunction with a bisulfite ion for increasing the brightness of a pulp by formation of a bleaching agent, such as hydrosulfite, An example of such prior efforts may be found in the U.S. Pat. No. 3,284,283 to Kindron et al, issued Nov. 8, 1966. To maintain the stability of an alkali metal borohydride, such as sodium borohydride, an alkaline solution of high pH is desirable. The required high pH of this alkaline solution, however, converts the bisulfite to sulfite so that in order to produce a bisulfite to hydrosulfite conversion the stability of the borohydride must be sacrificed and the solution pH lowered. This results in a complex and difficult process which does not fully utilize the expensive borohydride component. Examples of other previous efforts may be found in the following patents:
______________________________________ U.S. Pat. No. 2,069,943 Dreyfus February 9, 1937 2,996,422 Durant August 15, 1961 3,186,899 Madison June 1, 1965 3,100,732 Smedberg August 13, 1963 ______________________________________